Numerous modern electronic devices from the simple wrist watch to the more sophisticated computer servers depend upon the generation of one or more clock or oscillator signals. To meet the needs of various applications, the signals generated must be accurate and stable. In addition, the operational frequencies of the generated signals must not significantly deviate with changes in temperature from the design frequency.
Essentially all cell phones, computers, microwave ovens, and numerous other electronic products use a quartz crystal resonator to generate a reference signal at a pre-selected frequency which is typically around 20 MHz. Such oscillators are referred to as crystal-controlled oscillators. The gates in these products are “clocked” or switched at the pre-selected frequency using the reference signal. Any and all “time references” are generated from this quartz resonator-oscillator. In cell phones, laptop computers, and other portable devices, the quartz resonator-circuit is larger than desirable. Typically, the oscillator needs to have an approximate +/−2 ppm frequency drift over the product's full operational temperature range. To achieve this level of frequency control the quartz resonator is usually found packaged in a hermetic ceramic package with a metal lid that is arc-welded around the perimeter. As such, the package is relatively expensive. An example is the Kyocera TCXO part number KT21. This product is provided in a ceramic package that is 3.2×2.5×1 mm3, has +/−2 ppm accuracy from −30° to 85° C., and draws 2 mA of current. As this crystal's resonant frequency is 20 MHz, the signal from an oscillator using this product must be multiplied upwards by other power consuming electronics. Further, the resultant harmonics are generally only suppressed by approximately 5 dB relative to the fundamental frequency.
Oscillators can also be constructed using other types of resonators, for example standard L-C (inductor-capacitive) resonators, thin film bulk acoustic resonators (FBARS), and the like. While such resonators are less expensive than quartz resonators their frequency drift characteristics are generally less than acceptable for the applications mentioned above.